Measurements of two ultrasmall Josephson junctions connected in series show a supercurrent with magnitude much greater than previously reported for similar devices. The dependence of the supercurrent on gate charge shows agreement with predictions made by Averin and Likharev that are based on a conjugate relationship between the junction phase and charge variables. The transport of Cooper pairs through the two junctions makes the devices behave effectively as a single junction whose critical current is modulated by the gate charge.

Computer simulation is used to study Andreev reflection in nanoscale normal-metal–superconductor devices. Back-focusing of hole waves through quantum point contacts is demonstrated. The electron-hole conversion efficiency is shown to depend on the diffraction of the electron wave at the quantum point contact.

We study the phase diagram for the attractive (i.e., negative-U) Hubbard model on a simple-cubic lattice, through Monte Carlo simulations. We obtain the critical temperature ${\mathit{T}}_{\mathit{c}}$ for superconductivity from a finite-size scaling analysis of the data for the pairing correlations. For fixed on-site attraction U, ${\mathit{T}}_{\mathit{c}}$ displays a maximum near the filling factor 0.9, roughly independent of U. For fixed filling we estimate the crossover temperature ${\mathit{T}}^{\times }$(U), separating the normal states: metallic and spin gap. There is also a critical value ${\mathit{U}}_{\mathit{p}}$ for pair formation, the magnitude of which seems to be independent of doping. The relevance of these results to the high-${\mathit{T}}_{\mathit{c}}$ oxides is discussed.

The temperature dependence of the critical current ${\mathit{j}}_{\mathit{cm}}$(T) in single-crystalline ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ (BSCCO) changes dramatically with fast ion irradiation. In as-grown BSCCO crystals the ${\mathit{j}}_{\mathit{cm}}$(T) curve follows the predictions of the Feigelman-Vinokur theory for collective pinning in the presence of point defects. In irradiated BSCCO we have obtained a good fit of ${\mathit{j}}_{\mathit{cm}}$(T) over several decades and in a broad temperature range in the framework of the Nelson-Vinokur theory. The quality of the fit convincingly demonstrates the validity of this theory for the description of ${\mathit{j}}_{\mathit{cm}}$(T) in BSCCO with columnar defects.

The far-infrared transmission of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ thin films, grown on MgO substrates, was measured in the frequency range from 15 to 200 ${\mathrm{cm}}^{\mathrm{-}1}$, and temperature range from 9 to 100 K. There is a BCS-like peak in the transmission spectrum. The strong-coupling Eliashberg theory gives a better description of the optical data than the weak-coupling Mattis-Bardeen theory. For a ${\mathit{T}}_{\mathit{c}}$=18 K sample the Eliashberg fits give a London penetration depth of 5500±100 Å and an energy gap 2Δ(0)=4.0±0.1${\mathit{kT}}_{\mathit{c}}$ (6.2 meV).

The competition between magnetism and superconductivity was investigated for the recently discovered R${\mathrm{Ni}}_2$${\mathrm{B}}_2$C superconductors with the magnetic rare-earth elements R=Tm,Er,Ho,Dy. The systematic decrease of ${\mathit{T}}_{\mathit{c}}$, approximately scaled by the de Gennes factor, implies a very weak coupling between the rare-earth magnetic moments and the conduction electrons due to a small conduction-electron density at the rare-earth site. Associated with the antiferromagnetic order of the rare-earth moments, a pronounced dip structure in the upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) was observed; a similar structure has been seen in the previously known magnetic superconductors R${\mathrm{Rh}}_4$${\mathrm{B}}_4$ and R${\mathrm{Mo}}_6$${\mathrm{S}}_8$. For ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C, the pair-breaking associated with the magnetic transition is strong enough to bring about a resistive reentrant behavior even under zero field, an effect which, to our knowledge, has not previously been observed in antiferromagnetic superconductors.

${}_{}{}^{199}{}_{}{}^{}\mathrm{Hg}$ NMR spectra and ${\mathit{T}}_1^{\mathrm{-}1}$ are reported for two samples of ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ with superconducting transitions at ${\mathit{T}}_{\mathit{c}}$=95 K and ${\mathit{T}}_{\mathit{c}}$=80 K, respectively. The spectra are characteristic powder patterns with large anisotropic shifts: 3${\mathit{K}}_{\mathrm{ax}}$=-0.43% and -0.52%, respectively, for the two samples at room temperature with negligible temperature dependence below ${\mathit{T}}_{\mathit{c}}$. The isotropic Knight shifts were of order ${\mathit{K}}_{\mathrm{iso}}$=+0.11% with respect to a Hg(${\mathrm{NO}}_3$${)}_2$ aqueous solution. The relaxation rate is Korringa-like above ${\mathit{T}}_{\mathit{c}}$ while below ${\mathit{T}}_{\mathit{c}}$ the relaxation rate drops down exponentially without enhancement near ${\mathit{T}}_{\mathit{c}}$.

The spin- and density-correlation functions of the two-dimensional Hubbard model at low electronic density 〈n〉 are calculated in the ground state by using the power method, and at finite temperatures by using the quantum Monte Carlo technique. Both approaches produce similar results, which are in close agreement with numerical and high-temperature-expansion results for the two-dimensional t-J model. Using perturbative approximations, we show that the examination of the density-correlation function alone is not enough to support recent claims in the literature that suggested spin and charge separation in the low electronic density regime of the t-J model.

We have investigated the nonequilibrium critical dynamics of a two-dimensional, spin-exchange (i.e., conserved magnetization), kinetic Ising model following quenches to ${\mathit{T}}_{\mathit{c}}$ from random initial states. As in the case of subcritical quenches, we observe dynamical scaling of the equal time order parameter correlation function G(r,t,t), but here the nonequilibrium correlation length grows as L(t)∼${\mathit{t}}^{1/\mathit{z}}$, where z=4/15 is the usual dynamic critical exponent for this system. Moreover, we find dynamical scaling for correlations with the initial condition G(r,0,t). The temporal decay of autocorrelations at ${\mathit{T}}_{\mathit{c}}$ yields a critical exponent ${\lambda }_{\mathit{c}}$=1.96±0.05 for the nonequilibrium dynamics: G(r,0,t)≊${\mathit{L}}_{\mathit{c}}^{\mathrm{-}\lambda }$(t)G̃(r/L(t)).

We report here the ambient and high-pressure electronic-structure calculations of the potentially important ordered intermetallic alloy ${\mathrm{Zr}}_3$Al obtained using the self-consistent linear muffin-tin orbitals (LMTO) method. Further total-energy calculations were made using the tight-binding LMTO scheme in the ${\mathit{L}}_{12}$ and ${\mathit{DO}}_{19}$ phases in order to study its structural stability. From total-energy studies and the band filling of bonding states results, we conclude that it is easier to create instability in the ${\mathit{L}}_{12}$ lattice of ${\mathrm{Zr}}_3$Al and this prediction is consistent with the experimentally observed martensitic structural transition from ${\mathit{DO}}_{19}$→${\mathit{L}}_{12}$ structure. To make a comparative study, the band structure and total-energy calculations of isoelectronic ${\mathrm{Ti}}_3$Al were also made. The calculated bulk modulus and its pressure derivative, determined using a universal equation of state, are reported. The nature of chemical bonding in ${\mathrm{Zr}}_3$Al is also discussed. Using the band-structure results, within the BCS formalism, the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) was calculated and it is compared with the experimental value. The ambient-pressure band-structure, and ${\mathit{T}}_{\mathit{c}}$ calculations of the A15 compound ${\mathrm{Nb}}_3$Al were made with a view to compare its superconducting behavior with that of ${\mathrm{Zr}}_3$Al at high pressures as ${\mathrm{Zr}}_3$Al should mimic ${\mathrm{Nb}}_3$Al at high pressures. The pressure dependence of ${\mathit{T}}_{\mathit{c}}$ of ${\mathrm{Zr}}_3$Al is analyzed. Our calculations on ${\mathrm{Zr}}_3$Al are in contradiction with the often quoted correlation between the low density of states at the Fermi level and structural stability in similar systems and this is further corroborated with our more recent studies on ${\mathrm{Ti}}_3$In and ${\mathrm{Ni}}_3$In.

The electron-microscope surface-decoration technique and He-atom scattering (HAS) have been used to characterize the topography of NaCl(001) cleavage faces during vacuum ultraviolet (VUV) photon-stimulated desorption (PSD). The experiments were carried out at crystal temperatures below 550 K for which pure thermal desorption can be neglected. In the temperature range from T=550 to 440 K the decoration experiments reveal that PSD progresses mainly by the creation and subsequent spreading of two-dimensional (2D) holes of monolayer depth in the surface plane. From T=550 K down to 240 K the HAS experiments reveal periodic oscillations of the specularly scattered He intensity with exposure. These oscillations are attributed to the same process of creation and subsequent spreading of 2D holes which leads to a layer-by-layer removal of the surface. The HAS results indicate that the rate of PSD decreases strongly with decreasing temperature whereas the density of photon-induced 2D holes increases. The evolution with time of the specular He intensity also indicates that isolated adparticles are created during VUV irradiation. Simultaneous mass-spectrometer studies of the desorbing species reveal in addition to Na and Cl atoms a considerable PSD flux of NaCl molecules not predicted by the currently accepted models of PSD. Moreover the photon-induced desorption fluxes of the different species are found to be independent of the actual step density. Thus, the observed temporal alterations of the surface topography (step density) have no effect on the PSD process but solely reflect its progress.

A series of diffuse-neutron-scattering measurements on ${\mathrm{C}}_{60}$ powder material is presented. The results give evidence for the presence of significant static disorder below the 260-K ordering transition where besides the well-known simple cubic Bragg peaks elastic diffuse scattering is observed. Additional measurements on a small ${\mathrm{C}}_{60}$ crystal likewise show elastic diffuse scattering below the ordering temperature.

Size effects on the Curie temperature and spontaneous polarization in spherical particles are studied using Landau phenomenological theory. The extrapolation length is shown to be size dependent. The spatial distribution of polarization is obtained. The size dependence of the polarization is calculated whereby a size-driven phase transition is demonstrated. The Curie temperature as a function of particle size is calculated. Theoretical results are compared with the experimental in the literature and good agreement is obtained.

Three-photon excitation spectra of the lowest exciton of NaCl and KBr have been measured up to 6 kbar at low temperature. The experimental results on NaCl near 8.45 eV represent the deepest uv state studied to date under hydrostatic pressure. The exciton energy in both materials shows a linear blue shift with increasing pressure. In order to better understand the experimental results, the exciton energies were computed using the potential induced breathing model on the basis of the charge-transfer exciton model. We find evidence supporting the charge-transfer exciton model and show that the crystal-field stabilization of the anions and overlap energies which were neglected in previous models are quantitatively significant.

We present single-crystal data of mixed-valent ${\mathrm{YbInCu}}_4$ including a description of the single-crystal-growth procedure. Many of the data, such as magnetic susceptibility, electrical resistivity, and lattice constant, are similar to previously published polycrystalline data although the mixed-valent transition at 66.9 K is sharper than in the latter case. In addition, elastic-constant data are presented which exhibit a pronounced softening of the bulk modulus and the Poisson ratio at ${\mathit{T}}_{\mathit{a}}$.

A rate kinetic model to explain the general behavior of lyoluminescence has been recently proposed. The present work demonstrates another example of the validity of the model in explaining the observed effect of lyoluminescence from heat-treated samples. The samples after irradiation with γ rays were kept molten at their melting points for different periods of time before dissolution. The light yield on dissolution was found to decrease with samples kept molten for longer periods of time. It has been shown that the observed effect can be explained when changes in the rate of reaction mechanism and diffusion of oxygen due to heating are incorporated in the model.

Raman spectra of solid iodine were measured up to 26.8 GPa. New bands observed above 10 GPa are discussed within the framework of a previously suggested quasi-one-dimensional molecular phase existing prior to molecular dissociation. Higher-order spectra due to two- and three-phonon processes were observed up to 10 GPa, and provide information on the pressure dependence of phonon states near the edge of the Brillouin zone.

Cultured quartz crystals heavily doped with Ge have been examined for their near-infrared absorption bands at 77 K in the 3100–3700 ${\mathrm{cm}}^{\mathrm{-}1}$ range. The crystals investigated were grown slowly or fast, and also at high pressure. The Ge levels varied from 1000 to 3000 ppm. Ge doping has been found to exhibit the presence of near-infrared bands at 3481 and 3656 ${\mathrm{cm}}^{\mathrm{-}1}$ in addition to the conventional growth-defect bands. The band at 3656 ${\mathrm{cm}}^{\mathrm{-}1}$ was observed only at high Ge levels (∼3000 ppm) and was better resolved in the slowly grown material. A band at 3507 ${\mathrm{cm}}^{\mathrm{-}1}$ was observed in crystals grown fast and under high pressure. Irradiation at 77 and 300 K was performed using a Van de Graaff electrostatic accelerator. Bands at 3506 ${\mathrm{cm}}^{\mathrm{-}1}$ upon 77 K irradiation and 3525 ${\mathrm{cm}}^{\mathrm{-}1}$ after 300 K irradiation were noticed. The results are discussed in terms of fundamental considerations about the quartz crystal structure.

The study of a system of continuous spins on a frustrated triangular lattice evolving as a cellular automaton is presented. Initially disordered lattices are seen to organize into modulated patterns. It is shown that frustration on the triangular lattice is not a sufficient condition for the appearance of an incommensurate modulation; anisotropy, which breaks the hexagonal symmetry, must also be introduced. There is a critical value ${\mathrm{\beta }}_{\mathit{c}}$ for the anisotropy above which the incommensurability is found.